The present invention relates to a pneumatic tire provided with a plurality of main grooves extended in a tire circumferential direction on the tread surface. More particularly, the present invention relates to a pneumatic tire capable of controlling uneven wear occurring in the vicinity of the main grooves.
Generally, on the tread surface of the pneumatic tire, a single or plural curvature radius (tread radius), as a curvature, in a tire meridian direction is imparted based on the inner surface shape of a die mold. Meanwhile, in the pneumatic tire, the curvature radius on the tread surface tends to be changed during pressurization due to its inner structures such as a belt layer buried inside the tread portion, tread rubber and the like. When a change in curvature radius on the tread surface occurs in the above manner due to inflation, this change is absorbed by the groove portion, resulting in the occurrence of a phenomenon such as bending of the tread portion at the groove bottom as a boundary. Particularly, in a rib tire provided with a plurality of main grooves extended in the tire circumferential direction, the above bending phenomenon is significant.
As described above, when the tread portion is bent at the groove bottom as a boundary, the edge portion of the rib adjacent to the groove does not match the specified curvature radius of the tread surface, and the ground-contacting pressure in the rib edge portion is significantly changed. As a result, uneven wear occurs with the rib edge portion as a starting point, resulting in a growth of railway wear.
The object of the present invention is to provide a pneumatic tire capable of effectively controlling uneven wear occurring in the vicinity of a main groove even in the case where the groove width of the main groove is narrowed due to a change in the tread radius during inflation.
A pneumatic tire of the present invention for achieving the foregoing object is a pneumatic tire provided with a plurality of main grooves extended in a tire circumferential direction on a tread surface, characterized in that, with regard to some of the main grooves among the above plurality of main grooves whose groove width is narrowed during inflation, both groove walls are inclined so that the groove width becomes wider toward the groove bottom, and a protrusion dividing a groove space in the tire width direction is provided at the groove bottom. The protrusion comprises a flat top surface and a pair of slanted walls so that the protrusion has a generally trapezoidal shape.
As described above, with regard to the main groove having the groove width narrowed due to the change in tread radius during inflation, by making both of the groove walls inclined so that the groove width becomes wider toward the groove bottom, it is made possible to control the significant change in ground-contacting pressure in the rib edge portions at both sides of the main groove. Moreover, by providing a protrusion dividing the groove space in the tire width direction on the foregoing groove bottom, bending of the tread portion due to the change in tread radius is dispersed onto two points of both sides of the protrusion; therefore, the ground-contacting pressure on the both sides of the main groove can be equalized. Furthermore, it is made possible to absorb, into the protrusion, frictional energy acting so as to cause uneven wear on the rib edge portions of both sides of the main groove. Hence, uneven wear such as railway wear occurring in the vicinity of the main groove can be effectively controlled by the synergy of the inclined structure of both groove walls of the main groove and the protrusion.
In the present invention, if the height of the protrusion is made equal to or lower than the tread surface with a height difference between the top face of the foregoing the protrusion and the foregoing tread surface ranging from 0 to 2 mm, the absorption effect of the frictional energy by the protrusion can be increased during the initial wear. Particularly, the ratio of the protrusion height to the groove depth is preferably set at 0.8 or higher.
The protrusion can be divided in the tire width direction by providing a cut extended in the tire circumferential direction. In this case, the protrusion is better able to absorb the frictional energy. Moreover, a rubber composition constituting the protrusion and a rubber composition constituting the tread surface may be made different from each other. If a rubber composition with a grip property superior to that of the tread surface is selected as a rubber composition of the protrusion, it is made possible to supplement the frictional force of the tire on a wet road surface because of the grip property of the protrusion, even if the volume of the main groove is reduced as the wear progresses.